1. Field of the Invention:
The invention is directed to a method for the manufacture of a preform for drawing glass fibers, particularly infrared light waveguides for optical communications technology, by casting molten glass onto a pre-defined body of solid glass.
2. Description of the Prior Art
Infrared light waveguides for optical communications technology are manufactured of oxygen-free, heavy metal fluoride glasses. ZrF.sub.4, HfF.sub.4 or TlF.sub.4 are network-forming agents in these glasses. BaZr.sub.2 serves as network converter. The center of gravity in the glass stability lies close to the composition barium fluoro-diz-zirconate BaZr.sub.2 F.sub.10. For increasing the glass stability, doping is usually carried out with tri-valent fluorides of the rare earths, for example, with LaF.sub.3, GdF.sub.3, YbF.sub.3 or with YF.sub.3, AlF.sub.3, for example, with 0 through 20 Mol-percent. For example, a typical glass composition is 57 ZrF.sub.4 .multidot.34 BaF.sub.2 .multidot.5 LaF.sub.3 .multidot.4 AlF.sub.3. In order to suppress the crystallization tendency, alkkali fluoride, for example, NaF or LiF is also frequently added instead of a part of the BaF.sub.2. The general employment of BaF.sub.2 as glass component is characteristic.
Fibers of fluoride glass having a stepped refractive index profile are currently manufactured almost exclusively based on the preform method. A glass rod having a core of core glass and a jacket of jacket glass is thereby first prepared, this then being drawn thin to form a fiber. Typical of such glass rods are an outside diameter of 12 mm corresponding to the exterior jacket diameter, a core diameter of 10 mm, a length of 150 mm and a refractive index difference between the core glass and the jacket glass of 0.5 through 1%. The fibers drawn from such a rod have typical outside diameters of 0.1 through 0.2 mm.
As a consequence of the barium content of the glasses, the known CVD methods for the manufacture of preforms of silica glass cannot be applied here. On the contrary, one proceeds via the molten phase which must then be very quickly cooled so that the material solidifies vitreously and crystallization is avoided. The jacket glass is therefore first cast into a cylindrical preform which is usually fabricated of gold-coated brass. By rotation of the form, one then succeeds in forming a tube of jacket glass into which molten core glass can be subsequently filled. This method is known under the term "rotational casting" (cf., in this regard, D.C. Tran, C.F. Fisher, G.H. Sigel, Jr., "Fluoride Glass Preforms Prepared By A Rotational Casting Process", Electronics Lett. 18 (1982) Pages 657-658 and U.S. Pat. No. 4,519,826).
However, crystals or bubbles at the glass boundary surface between core and jacket are frequently found in preforms manufactured in this way. The cause thereof can probably be seen in the aging of the non-cooled inside surface of the jacket glass tube whereby processes such as the absorption of humidity from the atmosphere or the evaporation of melt components participate.
In a second method, the tiltable hollow form is first filled with molten jacket glass and is in turn partially emptied shortly thereafter by being turned over or rotated. This is possible because the molten jacket glass solidifies proceeding from the outer edge toward the inside. Molten core glass is filled in immediately thereafter.
This method is known under the term "build-in casting" (see S. Mitachi, T. Migashita, T. Kanamori, "Fluoride Glass Cladded Optical Fibres For Mid-infrared Ray Transmission", Electron. Lett. 17 (1981) pages 591-592). In this method, the boundary surface is usually less disturbed, but the core diameter greatly deviates from the desired cylindrical form instead.
What the two known methods have in common is the principal of first manufacturing a tube of jacket glass which is then filled out with molten core glass. The heating of the enveloping glass by the core glass can thereby not be avoided. This likewise promotes the crystal growth in the jacket glass. The thermal resistance of the enveloping glass is unfavorable in view of the manufacture of thicker preforms because this prevents the rapid cooling of the central volume.